This invention relates to a turbine control system which electro-hydraulically controls a turbine with a plurality of control valves, and more particularly to improved apparatus for controlling the opening degrees of the control valves of the turbine.
In general, in the control system of a steam turbine in which steam at high pressure and high temperature is introduced from a steam producing device to the steam turbine through a plurality of control valves, the flow-rate of the steam is controlled by operating these control valves to control the speed and output of the steam turbine.
Especially, in starting the turbine, all of the steam control valves are subjected to a so-called "throttle governing control" in which the closed steam control valves are gradually opened until the output of the turbine reaches a predetermined value, and thereafter to a so-called "nozzle governing control" in which the steam control valves are successively fully opened according to the desired output of the steam turbine. For this purpose, that is, in order to switch over the throttle governing operation to the nozzle governing operation, conventional steam turbines are provided with a control system for changing the opening degrees of the steam control valves.
In such a control system, in order that the steam control valves can maintain the desired operating performance of the turbine during operation, the "opening degree" -- "steam flow-rate" characteristic of each steam control valve is corrected with the aid of a main control flow-rate request signal delivered from a speed control section or a load control section in a manner such that the opening-degree of each steam control valve is changed to a suitable value through a valve control section.
However, it should be noted that even if one and the same main control flow-rate request signal is applied to the turbine, the opening degree characteristic of each steam control valve in the throttle governing operation is different from that in the nozzle governing operation. Accordingly, when the operating condition of the turbine is quickly changed from the throttle governing control condition to the nozzle governing control condition, some of the steam control valves abruptly increase their opening degrees, and therefore steam at high temperature and high pressure rapidly flows into the turbine therethrough, thereby imparting thermal shocks to the nozzle box, the turbine casing, etc., which may cause serious damage to the turbine.
In order to overcome such difficulty accompanying the conventional steam turbines a technique has been disclosed in U.S. Pat. No. 3,688,095 (corresponding to Japanese Pat. No. 627,126). In this technique, an analog control circuit includes contact means and is therefore inevitably intricate, and during the valve-opening-degree changing operation the above-described thermal shock is liable to occur depending on the offsetting conditions of an amplifier and other elements included therein. Furthermore, in this technique the flow-rate of steam of the turbine is maintained unchanged before and after the valve-opening-degree-changing operation; however, during this valve-opening-degree changing operation the steam flow-rate is varied because no control to keep the steam flow-rate constant is provided. Thus, it is impossible to eliminate the variation in output of the turbine by the proposed technique.